Miller's concept of electrophilic chemical carcinogenesis will be studied in case of four selected classes of significant pro-carcinogens: 1) Polycyclic aromatic hydrocarbons 2) Aromatic amines and nitro compounds 3) Nitrosoamines and related compounds. The nature of the active electrophilic agents will be determined under conditions allowing observations of stable long-lived carbocationic intermediates with methods previously developed in our laboratories i.e. 1H, 13C nmr ESCA (photoelectron) spectroscopy. The alkylating ability of carbocationic electrophiles formed by chemical activation of pro-carcinogens will also be studied particularly in case of nitrosamines. Under electrophilic conditions they were found to readily form immonium ions, which display (by cmr spectroscopy) ambident character showing significant contribution of their aminocarbenium ion forms.